Photo-conductive targets for cathode ray devices



May 8, 1956 s. v. FORGUE ErAL PHOTO-CONDUCTIVE TARGETS FOR CATHODE RAY DEVICES 2 Sheets-Sheet 1 Filed June l. 1951 i A RNEY PHOTO-CONDUCTIVE TARGETS FOR CATHODE RAY DEVICES Filed June l 1951 May 8, 1956 s. v. FORGUE ETAL 2 Sheets-Sheet 2 pw web mu Mor n V 1M M m www m TH E ME yang f @V e Yv//V M MU B p www C@ AN L10. m wm SRM p w W United States Patent O PHOTO-CONDUCTIVE TARGETS FOR CATHDE RAY DEVICES Stanley V. Forgue and Robert R. Goodrich, Granbury, N. J., assignors to Radio Corporation of Aamerica, a corporation of Delaware Application June 1, 1951, Serial No. 229,428

16 Claims. (Cl. 313-65) This invention relates to light-sensitive targets for electron discharge devices, and, more particularly, to photoconductive targets for television camera tubes and their method of manufacture.

Television camera tubes employing photo-conductive targets and known as Vidicons are described in an article beginning on page 70 of the May 1950 issue of Electronics Magazine and in copending U. S. patent application of S. V. Forgue, Ser. No. 198,130, led November 29, 1950.

A Vidicon camera tube consists of an electron gun and a target assembly contained in a glass envelope approximately six inches long and one inch in diameter. The electron gun is of the conventional type used in the image orthicon and other television pickup tubes. The target assembly comprises a film of light-transparent, electrically-conductive material on the glass face plate of the envelope, and a layer of photo-conductive material deposited upon the electrically-conductive film. The target and the gun are so arranged within the envelope that the electron beam from the gun scans the photoconductive surface of the target.

The photo-conductive material used for Vidicon targets is an electrical insulator in the dark, but becomes electrically-conductive when light is shed upon it. The conductivity is proportional to the amount of light striking the material, and is limited to the immediate area under the influence of the light.

Vidicons may be operated at either high or low velocity. That is, they may be operated with the target at suiiicient voltage positive with respect to the cathode so that the electron beam strikes the target with enough force to drive secondary electrons from the photo-conductive material, thereby rendering it more positive. Or, they may have cathode and target at approximately the same potential so that the scanning electron beam deposits electrons on the target with a negligible amount of secondary emission, thereby making the target more negative.

In one type of high velocity operation, the electron beam scans the photo-conductive surface facing the electron gun at a Velocity above first crossover, i. e. at a suiiiciently high velocity so that the number of secondary electrons leaving the surface will be greater than the number of primary electrons which arrive at the surface and cause the secondary emission therefrom. The first crossover potential differs with different target materials and surface conditions. Two hundred volts between target and cathode has given satisfactory results with antimony tri-sulfide targets.

This target potential is applied to the electrically-conductive film in contact with the opposite side of the photo-conductive layer from the one scanned by the electron beam. A collector potential, about ten volts positive with respect to the target is applied to a metal screen or ring, which is termed the collector electrode and is located immediately in front of the scanned surrice face of the photo-conductive layer. When the high velocity beam scans this surface, the resulting secondary electrons are attracted to the collector electrode making the scanned surface more positive until it reaches the potential of the collector electrode and equilibrium is established between them. When the scanning electron beam has brought the scanned surface to collector potential, it makes that surface ten volts positive with respect to the conductive film on the other side of the photo-conductive layer.

When a light image is focused upon the target, the photo-conductive material becomes conductive in the areas where the light impinges upon it. The effect of this conductivity is to cause a leakage of electrons from the electrically-conductive lm, through the photo-conductor, to the scanned surface. The amount of leakage depends upon the intensity of the light incident upon each area; and its effect is to make the area of the scanned surface affected by the light a volt or so more positive. When the scanning electron beam re-scans an area from which the electron charge has leaked, it restores the ten volt difference between the surfaces of the photo-conductor. This leakage and restoration causes an electron current iiow in the circuit between the light-transparent, electrically-conductive lilrn in contact with the photo-conductive target and the source of potential to which it is connected. Variations in the electron current through this circuit, as more or fewer electrons are needed to restore the ditierence in potential become the signal output of the tube.

In low velocity operation, the electrically-conductive tilm is connected to a source of potential approximately ten volts positive with respect to the cathode which is at ground potential. The electron beam scans the photoconductive surface, and, by depositing electrons thereon at a velocity less than first crossover (i. e. where the ratio of secondary electrons leaving the surface when primaries strike it is less than unity), brings the bombarded surface to cathode potential and produces approximately a ten volt difference of potential across the target.

When light impinges upon an area of the target, it renders the photo-conductive material conductive in the particular area upon which it impinges, and causes the corresponding portion of the scanned surface of the photo-conductor to give up electrons and come a volt or so closer in potential to the conductive film. The next time the electron beam scans this area it restores to cathode potential the area from which electrons have leaked under the light-induced conductivity. This return to cathode potential restores the ten Volt difference across the target and causes an electron current to the conductive lm from the source of potential to which the film is electrically connected. This electron current through an output resistor provides the signal output from the tube.

Some of the qualities of photo-conductive materials which must be considered in determining their desirability as Vidicon targets are: sensitivity, resistivity in the dark, lag, useful life, and current-to-light response.

Sensitivity has reference to the ability of the material to become conductive under the influence of light. It is measured in micro-amperes of video current output per lumen of light on the target.

Resis'tivity in the dark has reference to that quality of photo-conductive material which enables it to store an electrical charge in a given spot without leakage from front to back surface as long as there is no light on the target.

By lag is meant rapidity of response of the target to changes in light, i. e. the ability of the target to erase a signal in a given period of time without showing a shadow or trail of light. The problems arising from lag become acute when a light-colored moving object is televised against a dark background.

Useful lite has reference to the hours of operation that can be expected from a target, and its ability to stand up under the handling etc. involved in manufacturing processes.

Current-to-light response indicates the range of changes in light intensity which can be covered within given limits of current output.

It is an object of the present invention to provide an improved photo-conductive target for a cathode ray device, and one which shall exhibit great sensitivity, high resistivity, low lag, long useful life, a large range of variations in light intensity for given current limitations, and one further characterized by its mechanical stability.

Another and related object of the invention is to provide an improved method of manufacture or such targets.

These and related objects are accomplished in a preferred embodiment of the invention by evaporating a layer of red antimony tri-sullide upon a glass surface which has been covered by an electrically conductive light-transparent film. The target assembly thus prepare-d is contained within a glass envelope with an electron gun assembly and the envelope is evacuated and sealed o. Buring evacuation, the assembly is degassed by placing that area of the envelope containing the gun within an RF heating coil. While this heating process is going on, the target assembly is kept cool by an air blast trained upon it.

H. Miller and I. W. Strange in an article entitled Electrical reproduction of images by the photo-conductive eect, in the proceedings of the Physical Society for 1938, volume 50, page 344, make reference to experiments with various photo-conductive materials. They precipitated the crystalline form of red antimony trisultide onto a conductive surface and reported fairly strong signals produced by far red and infra-red light.

The present inventors have discovered that when red antimony tri-sulfide is heated to more than about 225 C. it undergoes a change of state from the red form to the black form and takes on difierent electrical characteristics, such as lower sensitivity, which make it less desirable as a camera tube target material. They have further discovered that the lag characteristics of the target are due not only to qualities inherent in the target material, but also to the capacitance between the scanned surface of the target material and the electrically-conductive hlm on its opposite side. By carefully following the procedure to be described, they have evaporated red antimony tri-sulde targets of suficient thickness to overcome capacitative lag, and have prevented the red antimony sulfide employed from changing into the less desirable black form.

The invention is explained in more detail by reference to the accompanying two sheets of drawings wherein:

Fig. l is a sectional view, partly diagrammatic, of apparatus used to prepare a photo-conductive target for a cathode ray device after the manner of the invention;

Fig. 2 is a view in section, partly diagrammatic, of a cathode ray pickup tube with its target being cooled during the degassing process, after the manner of the invention;

Fig. 3 is a graph showing the current-to-light response of a photo-conductive target prepared in accordance with the invention;

Fig. 4 is a sectional View of a portion of the target end of the tube shown in Figure 2 with legends added.

The apparatus shown in Fig. l comprises a deinountable evacuation chamber l?. consisting of a glass bell jar resting upon a flat base l5. The base is equipped with a bushing 17 through which a pipe li communicates between the interior of the assembly ll and an evacuating pump assembly 2l. Base l5' is also provided with a plurality of vacuum tight electrical teri minals 23, 23', 24, 24', 2S, 2.5. These terminals provide means for carrying electrical current from external sources (not shown) to points within the evacuation chamber ll.

Resting upon the base l5 and within the space enclosed by the bell jar 13 is an inverted cup-shaped membei' 27 upon which stands the tube envelope 29 on whose face 3l the layer of photo-conductive target material 33 is evaporated. The red antimony tri-sulfide target material which is to be evaporated to form the layer or coating is contained in a crucible which is formed by twisting a tungsten wire into the conical spiral shown in the drawing and coating it with aluminum oxide. The f ee ends of this wire are connected to the terminals 2.5, 2S in the base Ilz", to permit electrical contact outside the evacuation chamber ll. Sleeves 37 of insulating material prevent these wires from short-circuiting against a cylindrical metal shield 39 which is inserted inside the tube envelope during the evaporation process to prevent the photo-conductive material from condensing any part of the envelope 29 other than the face plate 3l. An auxiliary heating coil 4l is located outside of the tube envelope 25" and close to the face plate 3l upon which the target material is to be evaporated. Current to activate this coil comes through the terminals 23, 23 in the supporting base l5.

Antimony tri-sulfide targets, in accordance with the invention, may be either evaporated directly onto the glass face plate of the tube envelope, or they may be evaporated onto a separate light-transparent, electrically-conductive surface, later to be mounted within a tube envelope. The invention will be described as applied to the situation where the evaporation takes place directly onto the face plate of the tube.

As a preliminary step, a glass bl or envelope 223 has a light-transparent, electrically-conductive coating of material such as the chloride or oxide of tin deposited on the inside surface of its face plate 3l. The coatmakes electrical contact with a conductive ring #ff-5 which passes through the wall of the envelope 29. In another preliminary step, the envelope 29 and the mounts which are going to become a part of the inished tube, or are going to be used in its processing, are given a preliminary baking at 450 C. to drive out occluded gases.

After the preliminary baking, the glass envelope 29 is mounted within the evacuation chamber il in a vertical position, resting upon the supporting member Z7 which is positioned over the pipe i9. An auxiliary metal ring if? is inserted between the cylindrical member 27 and the glass envelope 29 to determine the exact height that the tace plate 3l of the envelope Il@ will assume within the bell jar 113, and consequently the distance between the face plate 3l and the Crucible 35 which contains the antimony tri-sulfide to be evaporated. By varying the thickness of the ring 47, the distance between the crucible 35 and the face plate 3l may be adjusted to obtain targets of suitable thickness and even distribution, and also to prevent peeling. Good targets have been obtained when the Crucible 35 was spaced 1% from the face plate 3l.

After the envelope Z9 has been mounted within the bell iar 713, the chamber lll is evacuated to about 105 mm. of mercury.

An electric current is then applied to the terminals 23, 3 to which the auxiliary heater dl is connected` This current serves to cause the heater to bring the face plate 3l to a temperature of approximately 50 C. The reasons to this preliminary heating or" the glass face plate are two-fold. First, the heating cleans the surface of the face plate thereby enabling the evaporated coating to make better contact with it. Second, it prevents peeling due to differences in expansion and contraction of the glass plate 3l and the antimony tri-sulfide coating 33. The inventors, in the course of their experiments observed that peeling targets demonstrated either one of two. phenomena. Either, the antimony tri-sulfidehad lost its continuity as a continuous iayer and had. broken up into isolated islands or the supporting surface; or, the evaporated layer showed a tendency to separate into separate plates sliding together and overlapping one another. They recognized, therefore, that the peeling 1esulted from the glass supporting surface in some cases contracting to a greater extent or at a faster rate than the antimony tri sulfide coating, or in some cases to a less extent or at a slower rate; and they reasoned that, by a proper pre-heating of the class surface 3l together with a judicious control of temperature during the evaporation and further processing of the tube, they could prevent unequal expansion and contraction and thereby prevent peeling. According to one schedule which has produced satisfactory results, pre-heating the glass face plate 31 to about 50 C., before evaporation, as described above, and not allowing it to rise above about 60 C. during evaporation provides for a uniform contracting of face plate and photo-conductive layer and prevents peeling. (Another aid in preventing peeling is the preliminary baking at about 450 C. referred to above. This permits a closer bond between the evaporated mateiial and the conductive film.)

The temperature of the face plate and the target as sembly can be observed by means of a thermo-couple 32 held against the glass envelope 29 in the target area by means of a clamping ring 34. Electrical leads from the thermo-couple 32 pass out of the evacuated chamber 11 by way of vacuum-tight terminals 2d, 2d and make contact with temperature indicating instruments (not shown).

After the glass face plate 3l has been brought to a temperature of about 50 C., as explained above, the current to the auxiliary heating coil 41 is turned off, current is applied to the terminals 25, 25 leading to the Crucible 35, and the evaporating process begins. Evaporation is allowed to continue until the layer 33 of antimony tri-sulde condensing on the face plate 3i has been built up to the proper thickness. The thickness of this layer is determined by two factors. it must be thin enough to` permit light penetration and thick enough to prevent lag.

The lag in a target, which has been defined above as therapidity of response to changes in light, is due to two different phenomena. There is material lag, which is different for different photo-conductive materials; and there is capacitative lag, which results from the capacitance of each particular target and is a function of the area, the thickness and the dielectric constant of the target, according to the formula,

onsssKA (where: C=capacitance in micro-microfarads; K=the dielectric constant of the material which constitutes the target; A=the surface area of the target; and =the thickness of the target in centimeters).

The capacitative lag is aected by two other factors related to the capacity of the target. These are the amount of charge stored across the target in the dark and the extent towhich this charge leaks through the target and is dissipated under the influence of light. The charge stored on the 'target depends upon the current density of the electron beam which does the charging and may be expressed by the formula Q=LT1 where, Qzthe charge; I=the current of the scanning electron beam; and T=the timeit takes the scanning beam to make one sweep of the target, i. e. to scan one frame. Where, as in the conventional Vidicon camera tube, the scanning eiectron beam. has a current of approximately .l of a microampere, and the frame scanning time is m of a second, the charge stored by the scanning beam in a single frame is l0 7 %,0 or 3 l09 coulombs. When light is focused upon the target, electrical conductivity is established through it; and the scanned surface of the photo-conductive layer swings approximately one volt closer to the-.potential of the other surface of the photo-conductive layer which is in contact with the conductive hlm.

When the electron beam scans its next frame, it restores the original potential across the target. This restoring of the original potential causes an electron current flow through the output resistor to the conductive [ilm and produces an output signal. lf the scanning electron beam does not have enough current to restore the charge which has been lost, or if the capacity of the target prevents the charge from being restored in a single frame time, a signal is generated in the output circuit for several frames after the light has been removed from the target. This signal shows up as a shadowy after-image on the viewing screen in a television system.

As explained above, the electron beam in a conventional Vidicon deposits approximately 3x109 coulombs of electrical charge in a l/o second frame time. Where Q=the amount of charge in coulonibs, Czthe capacitance in farads, and E=the voltage in volts, Q=CE or Substituting for the above the values already given This means that an ideally responsive target with no visible` lag' would have a capacitance of approximately 3X 10-9 farads or 3000 micro-microfarads.

To convert this capacitance value into terms of target thickness, use the formula set forth above.

If C (the desired capacitance in micro-microfarads): 3000 micro-microfarads, K (the dielectric constant of red antimony tri-sulfide) :10, and A (the surface area of the photo-conductive layer)=3 sq. cm. (presurning a target of approximately 2 cm. diameter), then d (the thickness of the dielectric, i. e. photoeonductive,

layer) C, 300g .000885 cm.

pousse 2.54

The above mathematical analysis demonstrates a method for determining theoretically the optimum target thickness. Satisfactory results for many purposes have, however, been obtained with targets .l of a mil thick and with a capacitance of 4000 micro-microfarads.

The thickness of target may be measured in several ways. A rough approximation is made by controlling the time of the evaporation process. If this is done, an evaporation time of about one-half hour produces a target of the proper thickness. Another method of determining proper target thickness is to count the interference fringesfrom the evaporated layer as it is illuminated during its formation by a monochromatic light, such as a sodium lamp. Still another method, is to use a previously fabricated target of satisfactory thickness as a guide and compare densities by means of parallel light beams. When the evaporation process has brought the target in process to the same optical density as the standard or reference target the evaporation process is terminated.

After the electrical current to the evaporating Crucible 35 has been discontinued the entire assembly is allowed to cool to room temperature before further processing. This usually takes from fifteen minutes to a half hour. Care should be taken to allow for coasting by, as far as target thickness is concerned, during this cooling period. Since there will be some residual heat after the current to the crucible 35 has been turned olf, there will be some :.00035 inch= .85 mil 7 further evaporation of the target material. Therefore, the current to the crucible 35 is turned 01T shortly before the photo-conductive layer 33 reaches its desired thickness.

It should be noted that in the above process, there is no interruption of the vacuum between the preliminary heating of the glass face plate 31 and the subsequent evaporation of the photo-conductive layer 33. This ternperature is controlled by means of a variac (not shown in the drawings) in the current supply to the crucible 35.

After the face plate 31 and evaporated layer of photoconductive material 33 have cooled to substantially room temperature, the envelope 2@ is removed from the vacuum for further processing of the tube. A screen electrode l9 is inserted into the tube and welded to a conductive ring Si which passes through the wall of the envelope 29 (see Fig. 2).

The electron gun assembly 53 is inserted into position within the envelope and the tube is put onto the evacuating pumps for the sealing process. the process, an RF coil 55, for degassing purposes, is placed around the tube in the vicinity of the electron gun assembly 53 and bakes that assembly for one to one and one-half hours at a temperature of about 150 C. to 175 C. Care must be taken that the photo-conductive film 53 of red antimony tri-suliide never is eX- posed to a temperature of more than 225 C. to prevent it from converting to the undesirable black form. In order to keep the target cool, and at the same time to permit the gun assembly to be heated to the proper temperature for successful degassing, an air blast from an external source 57 is played upon the face of the target during the sealing and degassing operation.

Photo-conductive targets prepared in the manner described have been found to be much more sensitive than similar targets previously known to the art. Sensitivities of several thousand micro-amperes per lumen have been noted in some targets, and several hundred micro-amperes per lumen is quite common. These targets have been found to have useful lives of several thousand hours at temperatures even above those likely to be encountered in normal television camera operation. And they have good qualities of resistivity and lag.

Also, as shown in Fig. 3, they aiord approximately a square root current-to-light response. While a material with a linearresponse (plot A) could cover only a little more than three units of shading in light intensity for the tenth of a micro-ampere change in current between .15 and .25 micro-ampere, a target prepared i1 accordance with the invention (plot B) could cover sixteen such units. in practical results this means that these targets make it possible to reproduce more delicate shadings of light in a television system.

What is claimed is:

l. A photo-conductive target for a cathode ray device comprising a supporting member, a layer of electricallyconductive material, and a layer of evaporated red antimony tri-sullide.

2. A cathode ray tube including an evacuated envelope containing an electron gun assembly and a light-sensitive target assembly, said target assembly comprising: a supporting member, electrically conductive coating on said supporting member, and a layer of evaporated red antimony tri-sulfide on said conductive coating.

3. The invention according to claim 2 and wherein said layer of evaporated red antimony tri-sulfide is more than .l mil thick.

4. The invention according to claim 2 and wherein the capacitance of said layer is less than 4,000 micro-microfarads.

5. The invention according to claim 1 and wherein the capacitance of said target is approximately 3,000 micromicrotarads.

6. A light sensitive target assembly for a cathode ray tube comprising, a supported member, a layer of evap- During this part of orated red antimony tri-sulfide on said support member substantially of a thickness determined by the formula 0.0885KA C' Where K is the dielectric constant or" red antimony trisulfide, A is the area of the antimony tri-sulfide layer in square centimeters, and C is the desired capacitance of the target in micro-microfarads.

7. A light sensitive target assembly for a cathode ray tube comprising, a support member, a layer of evaporated red antimony tri-sulfide on said support member whose surface area and thickness are in such relationship With each other that where K is the dielectric constant of red antimony trisulde, A is the area of the antimony tri-sulfide layer in square centimeters, and D is the thickness of the red antimony tri-sulfide layer in centimeters.

8. A light sensitive target assembly for a cathode ray tube comprising, a support member, a layer of evaporated red antimony tri-sulfide on said support member whose surface area and thickness are in such relationship with each other that 0.0885KA D where K is the dielectric constant of red antimony trisulde, A is the area of the antimony tri-sulfide layer in square centimeters, and D is the thickness of the red antimony tri-suitide layer in centimeters.

9. In a method of preparing, for an electron discharge device, a light-sensitive element containing red antimony tri-sulfide, the step which comprises cooling said element during the processing of said device so that said element does not rise above the temperature at which red antimony tii-sulide converts to the black form.

10. The method of making a photo-conductive target for a cathode ray camera tube, said target including a support element, said method including the steps of heating said support element in a vacuum, and evaporating a layer of red antimony tri-sulfide on a heated surface or said support element in said same vacuum.

1l. The method of making a photo-conductive target for an electron discharge device of the type having said target and an electron-gun assembly enclosed in a sealed envelope, said method comprising, evaporating red antimony tri-sulfide upon said target and cooling said target with an air blast while outgassing said gun assembly with heat.

12. The method of making a photo-conductive target for an electron discharge device which comprises: evaporating, in vacuo, red antimony tri-sulfide upon an electrically conductive foundation surface, and subsequently maintaining said red antimony tri-sulfide at a temperature of less than 225 C.

13. The method of preparing photo-conductive targets for cathode ray devices, said method including the steps of heating an electrically-conductive support structure in a vacuum, evaporating a coating of red antimony tri-sulfide on a heated surface of said structure, sealing said coated structure surface in an evacuated envelope containing an electron-gun, degassing said gun with heat during said sealing, and keeping said coated surface at a temperature below that at which red antimony tri-sulfide converts to the black form during said sealing and degassing.

14. The method of preparing a photo-conductive target for a cathode ray device which comprises: heating to approximately 50 C. in a vacuum an envelope having a glass race plate coated on its surface interior to said envelope a iayer of light-transparent, electrically-conductive material; evaporating from a source within said envelope a coating of red antimony tri-sulfide upon said surface,

D (thickness in centimeters)= :less than 4,000 miero-microfarads :3,000 micro-microfarads heating said target to a temperature of approximately 60 C. during said evaporation, allowing said coated surface to cool to room temperature, removing said envelope from said vacuum, mounting an electron gun in said envelope, evacuating and sealing said envelope and simultaneously de-gassing said electron gun with heat, and keeping said coated surface at a temperature below 225 C. by means of an air blast during said sealing and degassing.

15. A light-sensitive element comprising, a support member, a layer of evaporated red antimoney tri-sulfide on said support member, and a conducting element in contact with a portion of said evaporated layer.

16. A photoconductive element comprising, a support member, a conductive element on a surface of said support member, and a layer of evaporated red antimony trisulde in contact with said conductive element.

References Cited in the le of this patent UNITED STATES PATENTS 10 Gray Mar. 25, 1941 Klatzow Aug. 26, 1941 Gray Dec. 1, 1942 Janes June 11, 1946 Janes June 11, 1946 Schade July 16, 1946 Janes Nov. 25, 1947 Pfund June 16, 1953 Goodrich Oct. 6, 1953 Weimer Aug. 24, 1954 OTHER REFERENCES De Ment: Fluorochemistry, page 358, published 1945. Mellor: Comprehensive Treatise on Inorganic and 15 Theoretical Chemistry, page 591, vol. 9, published 1929. 

